gpg: Do not bail out on v5 keys in the local keyring.

* g10/parse-packet.c (parse_key): Return GPG_ERR_UNKNOWN_VERSION
instead of invalid packet.
* g10/keydb.c (parse_keyblock_image): Do not map the unknown version
error to invalid keyring.
(keydb_search): Skip unknown version errors simlar to legacy keys.
* g10/keyring.c (keyring_rebuild_cache): Skip keys with unknown
versions.
* g10/import.c (read_block): Handle unknown version.

When using gpg 2.3 the local keyring may contain v5 keys. This patch
allows the use of such a keyring also with a 2.2 version which does
not support v5 keys. We will probably need some more tweaking here
but this covers the most common cases of listing keys and also
importing v5 keys.

• Signed-off-by: Werner Koch <wk@gnupg.org>